Selection criteria

Studies containing analyses of the association between publication and the statistical significance or direction of the results (trial
findings), for a cohort of registered clinical trials.

Data collection and analysis

Two authors independently extracted data. We classified findings as either positive (defined as results classified by the investigators as
statistically significant (P < 0.05), or perceived as striking or important, or showing a positive direction of effect) or negative (findings
that were not statistically significant (P ≥ 0.05), or perceived as unimportant, or showing a negative or null direction in effect). We
extracted information on other potential risk factors for failure to publish, when these data were available.

Main results

Five studies were included. Trials with positive findings were more likely to be published than trials with negative or null findings (odds
ratio 3.90; 95% confidence interval 2.68 to 5.68). This corresponds to a risk ratio of 1.78 (95% CI 1.58 to 1.95), assuming that 41% of negative trials are published (the median among the included studies, range = 11% to 85%). In absolute terms, this means that if
41% of negative trials are published, we would expect that 73% of positive trials would be published.
Two studies assessed time to publication and showed that trials with positive findings tended to be published after four to five years
compared to those with negative findings, which were published after six to eight years. Three studies found no statistically significant
association between sample size and publication. One study found no significant association between either funding mechanism,
investigator rank, or sex and publication.

Authors' conclusions

Trials with positive findings are published more often, and more quickly, than trials with negative findings.